The present invention relates to a non-aqueous electrolyte secondary battery with a high energy density, and to a method for producing the same.
A non-aqueous electrolyte secondary battery using an alkali metal such as lithium or sodium in the negative electrode has a high electromotive force. Furthermore, the battery has a higher energy density than conventional nickel-cadmium storage batteries and lead-acid storage batteries. Above all, various researches have been carried out about non-aqueous electrolyte secondary batteries using Li in the negative electrode. However, the use of an alkali metal for the negative electrode causes dendrite during charging. This is likely to cause a short circuit, thereby lowering the reliability of the battery.
Therefore, it has been tried to use an alloy of lithium and either aluminum or lead for the negative electrode. The use of the alloy among them for the negative electrode makes lithium be absorbed in the alloy in the negative electrode during charging. As a result, no dendrite occurs, making the battery highly reliable. However, the discharge potential of the alloy is about 0.5 V higher than metallic lithium. Therefore, the voltage of the battery is lowered by 0.5 V, and the energy density of the battery is also lowered.
On the other hand, there has been a study on a negative electrode containing a layered compound comprising a carbon material such as graphite and lithium as an active material. Batteries with this negative electrode have been in a practical use as lithium ion secondary batteries. In the layered compound, lithium is intercalated between the carbon layers by charging, so that no dendrite is caused. Furthermore, the negative electrode has a discharge potential only about 0.1 V higher than metallic lithium, which causes only a minor voltage drop of the battery. However, when the carbon material is graphite, the upper limit of the amount of lithium to be intercalated between the carbon layers by charging is theoretically one sixth of the number of carbon atoms. In that case, the electric capacity is 372 Ah/kg. Thus, carbon material having lower crystallinity than graphite, various alloys and metal oxides which have larger capacities than the above have been put fourth.
Moreover, as the non-aqueous electrolyte secondary batteries have higher capacity and higher performance, it has been suggested adding a phosphate to the non-aqueous electrolyte in order to mainly improve the flame resistance and reliability of the non-aqueous electrolyte secondary batteries (Japanese Laid-Open Patent Application Hei 4-184870, Japanese Laid-Open Patent Application Hei 8-111238, Japanese Laid-Open Patent Application Hei 9-180721, and Japanese Laid-Open Patent Application Hei 10-55819 for example). To be more specific, it has been suggested to make the non-aqueous electrolyte contain a large amount of phosphate which is obtained by substituting aliphatic hydrocarbon groups having 4 or fewer carbon atoms for three hydroxyl groups in the phosphate.
The appearance of the negative electrode active material with a high capacity has realized a non-aqueous electrolyte secondary battery having a large discharge capacity; however, it consequently causes the following problems. That is, when capacity per unit weight or unit volume of the electrode increases, the non-aqueous electrolyte is decomposed on the electrode as the charge/discharge cycle proceeds, gradually lowering the discharge capacity. When a charged battery is stored at high temperatures, the non-aqueous electrolyte is decomposed or deteriorated on the electrode. As a result, the battery characteristics are impaired.
The present invention has an object of providing a non-aqueous electrolyte secondary battery in which a decrease in discharge capacity with the progress of a charge/discharge cycle and the deterioration of characteristics after the storage thereof at high temperatures are reduced by suppressing the reaction between the non-aqueous electrolyte and the electrode.
The present invention relates to a non-aqueous electrolyte secondary battery comprising a chargeable and dischargeable positive electrode, a non-aqueous electrolyte containing a lithium salt, and a chargeable and dischargeable negative-electrode, wherein at least one of the positive electrode, the non-aqueous electrolyte and the negative electrode contains at least one selected from the group consisting of a phosphate represented by the general formula (1): 
where R1a, R2a and R3a independently represent an aliphatic hydrocarbon group having 7 to 12 carbon atoms, a phosphate represented by the general formula (2): 
where R1b and R2b independently represent an aliphatic hydrocarbon group having 1 to 12 carbon atoms or an aromatic hydrocarbon group, and a phosphate represented by the general formula (3): 
where R1c represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms or an aromatic hydrocarbon group.
It is preferable that at least two selected from the group consisting of R1a, R2a and R3a are identical with each other in the general formula (1), and/or R1b and R2b are identical with each other in the general formula (2).
The present invention also relates to a non-aqueous electrolyte secondary battery wherein at least one of the positive electrode, the non-aqueous electrolyte and the negative electrode contains a mixture of at least two selected from the group consisting of a phosphate represented by the general formula; (1), a phosphate represented by the general formula (2), and a phosphate represented by the general formula (3), where R1a, R2a, R3a, R1b, R2b and R1c in the general formulae (1) to (3) have the same number of carbon atoms, respectively.
It is preferable that the percentage by volume of each of the phosphates in the mixture is not less than 30%.
It is particularly preferable that the mixture of phosphates is a mixture of the phosphate represented by the general formula (2) and the phosphate represented by the general formula (3), where R1b, R2b and R1c in the general formulae (2) and (3) have the same number of carbon atoms. It is also preferable that R1b, R2b and R1c are of the identical group.
The present invention further relates to a non-aqueous electrolyte secondary battery, wherein at least one of the positive electrode, the non-aqueous electrolyte and the negative electrode contains at least one phosphate selected from the group consisting of dibutyl phosphate, dipentyl phosphate, dihexyl phosphate, diheptyl phosphate, dioctyl phosphate, dinonyl phosphate, didecyl phosphate, diundecyl phosphate, didodecyl phosphate, monobutyl phosphate, monopentyl phosphate, monohexyl phosphate, monoheptyl phosphate, monooctyl phosphate, monononyl phosphate, monodecyl phosphate, monoundecyl phosphate and monododecyl phosphate.
Above all, it is preferable that at least one of the positive electrode, the non-aqueous electrolyte and the negative electrode contains a mixture of at least one selected from the group consisting of dibutyl phosphate, dipentyl phosphate, dihexyl phosphate, diheptyl phosphate, dioctyl phosphate, dinonyl phosphate, didecyl phosphate, diundecyl phosphate, and didodecyl phosphate, and at least one selected from the group consisting of monobutyl phosphate, monopentyl phosphate, monohexyl phosphate, monoheptyl phosphate, monooctyl phosphate, monononyl phosphate, monodecyl phosphate, monoundecyl phosphate and monododecyl phosphate. In that case, it is preferable that the phosphates in the mixture have the same aliphatic hydrocarbon group, like a mixture of dibutyl phosphate and monobutyl phosphate.
It is preferable that the non-aqueous electrolyte contains 0.1 to 20 wt % of the phosphate.
It is also preferable that the chargeable and dischargeable positive electrode contains at least one selected from the group consisting of LiCoO2, LiMn2O4, LiNiO2 and LiFeO2, and the chargeable and dischargeable negative electrode contains at least one selected from the group consisting of a carbon material, metallic lithium, a lithium alloy and a compound containing lithium. It is preferable that the lithium alloy contains at least one selected from the group consisting of Sn, Si, Al and In, besides lithium.
The present invention further relates to a method for producing a non-aqueous electrolyte secondary battery comprising the steps of: preparing an electrode mixture comprising an active material, a conductive agent and a binder, applying the electrode mixture on a current collector plate to prepare an electrode assembling a non-aqueous electrolyte secondary battery using the electrode and a non-aqueous electrolyte and, adding at least one of the active material, the electrode mixture and the electrode with at least one selected from the group consisting of a phosphate represented by the general formula (1): 
where R1a, R2a and R3a independently represent an aliphatic hydrocarbon group having 7 to 12 carbon atoms, a phosphate represented by the general formula (2): 
where R1b and R2b independently represent an aliphatic hydrocarbon group having 1 to 12 carbon atoms or an aromatic hydrocarbon group, and a phosphate represented by the general formula (3): 
where R1c represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms or an aromatic hydrocarbon group.
When the phosphate is added to the active material, the active material and the phosphate can be directly mixed, or the active material can be immersed in a solution containing the phosphate.
Furthermore, when the phosphate is added to the electrode mixture, the phosphate can be directly mixed with the electrode mixture.
Furthermore, when the phosphate is added to the electrode, the electrode may be immersed either in the phosphate or in a solution containing the phosphate.